In mobile communication networks heterogeneous architectures become more and more important. Heterogeneous Networks (HetNets) are networks, which utilize cell types of different sizes, as, for example, macro cells and small cells, such as metro cells, micro or pico cells, and femto cells. Such cells are established by base station transceivers for which their coverage areas are determined by their transmission power and interference condition. Small cells are cells having a smaller coverage area than macro cells. In some network scenarios the small cells' coverage area can be surrounded by the coverage area of a macro cell. The small cells can be deployed to extend the capacity of the network.
With regard to standardization, within the 3rd Generation Partnership Project (3GPP), HetNets have been added to the scope of the Long Term Evolution-Advanced (LTE-A) work item. Since the cells or base stations in such networks may utilize the same frequency resources, such architectures can suffer from interference created by the overlapping coverage areas of these cells. Therefore enhanced Inter-Cell Interference Coordination (eICIC) for co-channel HetNet deployment is one of the key techniques for LTE Release 10 (Rel-10). Co-channel HetNets comprise macro cells and small cells operating on the same frequency channel. Such deployments present some specific interference scenarios for which eICIC techniques are utilized.
In one example scenario, the small cells are open to users of the macro cell network. In order to ensure that such small cells carry a useful share of the total traffic load, User Equipment (UE) or mobile transceivers may be programmed or configured to associate preferentially with the small cells rather than the macro cells, for example, by biasing the Signal-to-Interference-and-Noise Ratio (SINR) or a Reference Signal Receive Power (RSRP) threshold at which they will select a small cell to associate with. Under such conditions, UEs near the edge of a small cell's coverage area may suffer strong interference from one or more macro cells. In order to alleviate such interference, some radio frames or sub-frames may be configured as “blank” or “almost blank” in a macro cell. A blank sub-frame may contain no transmission from the macro cell, while an “almost blank” sub-frame typically contains no payload data transmission and little or no control signaling transmission, but may contain reference signal transmissions in order to ensure backward compatibility with legacy terminals, which expect to find the reference signals for measurements but are unaware of the configuration of almost blank sub-frames. Almost blank sub-frames may also contain synchronization signals, broadcast control information and/or paging signals. The utilization of “blank” or “almost blank” sub-frames enables reduced or even suppressed interference for the small cell within these sub-frames. Hence, “blank” or “almost blank” sub-frames may be regarded as radio frames or sub-frames during which at least some radio resources are suspended from transmission, i.e. the transmission power of a cell or base station transceiver may be reduced on these radio resources.
Moreover, for coverage expansion, mostly for coverage of a small cell surrounded by one or more macro cells, within HetNets a bias value has been defined. For example, based on the bias value a mobile transceiver or User Equipment (UE) may request a handover from a macro cell to a small cell earlier than from one macro cell to another macro cell.
Another example scenario can arise with HetNets in which one or more cells operate on a Closed Subscriber Group (CSG) basis, and are therefore typically not open to users of the cellular network. For example, such a scenario can occur when CSG femto cells are being installed to cover households, but only allow a number of registered mobile to associate, while other mobiles are blocked. In this case, the small cells can cause strong interference to the macro cell UEs when these macro cell UEs come close to or enter the coverage area of the small cell CSG base station transceivers, however, without having the possibility to associate with them, i.e. to be handed over to them. It may then be beneficial for the open cells to indicate to their UEs the sub-frames in which they should make resource specific measurements, i.e. the sub-frames in which interference from one or more CSG cells is reduced or absent. In the following, to a base station transceiver may also be referred to as NodeB (NB) or as eNodeB (eNB) according to the 3GPP terminology.
However, to make use of blank or Almost Blank Sub-frames (ABSs) effectively (note that the term “ABS” is used, and should be understood to include both blank and almost blank sub-frames), signaling may be utilized between the cells, e.g. across the corresponding backhaul interface, known in LTE as the “X2” interface. For LTE Rel-10, it has been agreed that this X2 signaling will take the form of a coordination bitmap to indicate the ABS pattern (for example with each bit corresponding to one sub-frame in a series of sub-frames, with the value of the bit indicating whether the sub-frame is an ABS or not). Such signaling can help the cell to schedule data transmissions in the small cell appropriately to avoid interference (e.g. by scheduling transmissions to UEs near the edge of the small cell during ABSs), and to signal to the UEs the sub-frames, which should have low macro cellular interference and should therefore be used for measurements. Examples for such measurements are measurements for Radio Resource Management (RRM), which typically relate to handover, measurements for Radio Link Monitoring (RLM), which typically relate to detection of serving radio link failure, and measurements for Channel State Information (CSI) or Channel Quality Information (CQI), which typically relate to link adaptation on the serving radio link. In a CSG scenario, the ABS frames of the CSG cell can be used for scheduling data transmissions from other cells with reduced interference from the CSG cell.
In such an example scenario, Radio Resource Control (RRC) signaling can be utilized to indicate to the UEs the set of sub-frames which they should use for measurements (e.g., for RLM/RRM or CSI), where RRC is a signaling protocol standardized by 3GPP for control and configuration signaling.